The invention generally relates to the field of mechanical locking of floorboards. The invention relates to an improved locking system for mechanical locking of floorboards, a floorboard provided with such an improved locking system, a flooring made of such mechanically joined floorboards, and a method for making such floorboards. The invention generally relates to an improvement of a locking system of the type described and shown in WO 94/26999 and WO 99/66151.
More specifically, the invention relates to a locking system for mechanical joining of floorboards of the type having a body and preferably a surface layer on the upper side of the body and a balancing layer on the rear side of the body, said locking system comprising: (i) for horizontal joining of a first and a second joint edge portion of a first and a second floorboard respectively at a vertical joint plane, on the one hand a locking groove which is formed in the underside of said second board and extends parallel with and at a distance from said vertical joint plane at said second joint edge and, on the other hand, a strip integrally formed with the body of said first board, which strip at said first joint edge projects from said vertical joint plane and supports a locking element, which projects towards a plane containing the upper side of said first floorboard and which has a locking surface for coaction with said locking groove, and (ii) for vertical joining of the first and second joint edge, on the one hand a tongue which at least partly projects and extends from the joint plane and, on the other hand, a tongue groove adapted to coact with said tongue, the first and second floorboards within their joint edge portions for the vertical joining having coacting upper and coacting lower contact surfaces, of which at least the upper comprise surface portions in said tongue groove and said tongue.
The present invention is particularly suitable for mechanical joining of thin floating floorboards made up of an upper surface layer, an intermediate fibreboard body and a lower balancing layer, such as laminate flooring and veneer flooring with a fibreboard body. Therefore, the following description of the state of the art, problems associated with known systems, and the objects and features of the invention will, as a non-restricting example, focus on this field of application and, in particular, on rectangular floorboards with dimensions of about 1.2 m* 0.2 m and a thickness of about 7-10 mm, intended to be mechanically joined at the long side as well as the short side.
Thin laminate flooring and wood veneer flooring are usually composed of a body consisting of a 6-9 mm fibreboard, a 0.20-0.8 mm thick upper surface layer and a 0.1-0.6 mm thick lower balancing layer. The surface layer provides appearance and durability to the floorboards. The body provides stability and the balancing layer keeps the board level when the relative humidity (RH) varies during the year. The RH can vary between 15% and 90%. Conventional floorboards of the type are usually joined by means of glued tongue-and-groove joints (i.e. joints involving a tongue on a floorboard and a tongue groove on an adjoining floorboard) at the long and short sides. When laying the floor, the boards are brought together horizontally, whereby a projecting tongue along the joint edge of a first board is introduced into a tongue groove along the joint edge of the second adjoining board. The same method is used at the long side as well as the short side. The tongue and the tongue groove are designed for such horizontal joining only and with special regard to how glue pockets and gluing surfaces should be designed to enable the tongue to be efficiently glued within the tongue groove. The tongue-and-groove joint presents coacting upper and lower contact surfaces that position the boards vertically in order to ensure a level surface of the finished floor.
In addition to such conventional floors, which are connected by means of glued tongue-and-groove joints, floorboards have recently been developed which are instead mechanically joined and which do not require the use of glue. This type of mechanical joint system is hereinafter referred to as a xe2x80x9cstrip-lock systemxe2x80x9d, since the most characteristic component of this system is a projecting strip which supports a locking element.
WO 94/26999 and WO88/66151 (owner Vxc3xa4linge Aluminium AB) disclose a strip-lock system for joining building panels, particularly floorboards. This locking system allows the boards to be locked mechanically at right angles to as well as parallel with the principal plane of the boards at the long side as well as at the short side. Methods for making such floorboards are disclosed in EP 0958441 and EP 0958442 (owner Vxc3xa4linge Aluminium AB). The basic principles of the design and the installation of the floorboards, as well as the methods for making the same, as described in the four above-mentioned documents are usable for the present invention as well, and therefore these documents are hereby incorporated by reference.
In order to facilitate the understanding and description of the present invention, as well as the comprehension of the problems underlying the invention, brief description of the basic design and function of the known floorboards according to the above-mentioned WO 94/26999 and WO 99/66151 will be given below with reference to FIGS. 1-3 in the accompanying drawings. Where applicable, the following description of the prior art also applies to the embodiments of the present invention described below.
FIGS. 3a and 3b are thus a top view and a bottom view respectively of a known floorboard 1. The board 1 is rectangular with a top side 2, an underside 3, two opposite long sides 4a, 4b forming joint edge portions and two opposite short sides 5a, 5b forming joint edge portions.
Without the use of the glue, both the long sides 4a, 4b and the short sides 5a, 5b can be joined mechanically in a direction D2 in FIG. 1c, so that they join in a joint plane F (marked in FIG. 2c). For this purpose, the board 1 has a flat strip 6, mounted at the factory, projecting horizontally from its one long side 4a, which strip extends throughout the length of the long side 4a and which is made of flexible, resilient sheet aluminium. The strip 6 can be fixed mechanically according to the embodiment shown, or by means of glue, or in some other way. Other strip materials can be used, such as sheets of other metals, as well as aluminium or plastic sections. Alternatively, the strip 6 may be made in one piece with the board 1, for example by suitable working of the body of the board 1. The present invention is usable for floorboards in which the strip is integrally formed with the body and solves special problems appearing in such floorboards and the making thereof. The body of the floorboard need not be, but is preferably, made of a uniform material. However, the strip 6 is always integrated with the board 1, i.e. it is never mounted on the board 1 in connection with the laying of the floor but it is mounted or formed at the factory. The width of the strip 6 can be about 30 mm and its thickness about 0.5 mm. A similar, but shorter strip 6 is provided along one short side 5a of the board 1. The part of the strip 6 projecting from the joint plane F is formed with a locking element 8 extended throughout the length of the strip 6. The locking element 8 has an operative locking surface 10 facing the joint plane F and having a height of e.g. 0.5 mm. When the floor is being laid, this locking surface 10 coacts with a locking groove 14 formed in the underside 3 of the joint edge portion 4b of the opposite long side of an adjoining board 1xe2x80x2. The short side strip 6xe2x80x2 is provided with a corresponding locking element 8xe2x80x2, and the joint edge portion 5b of the opposite short side has a corresponding locking groove 14xe2x80x2. The edge of the locking grooves 14, 14xe2x80x2 facing away from the joint plane F forms an operative locking surface 10xe2x80x2 for coaction with the operative locking surface 10 of the locking element.
Moreover, for mechanical joining of both long sides and short sides also in the vertical direction (direction D1 in FIG. 1c) the board is formed with a laterally open recess 16 along one long side (joint edge portion 4a) and one short side (joint edge portion 5a). At the bottom, the recess 16 is defined by the respective strips 6, 6xe2x80x2. At the opposite edge portions 4b and 5b there is an upper recess 18 defining a locking tongue 20 coacting with the recess 16 (see FIG. 2a).
FIGS. 1a-1c show how two long sides 4a, 4b of two such boards 1, 1xe2x80x2 on an underlay 12 can be joined together by means of downward angling. FIGS. 2a-2c show how the short sides 5a, 5b of the boards 1, 1xe2x80x2 can be joined together by snap action. The long sides 4a, 4b can be joined together by means of both methods, while the short sides 5a, 5bxe2x80x94when the first row has been laidxe2x80x94are normally joined together subsequent to joining together the long sides 4a, 4b and by means of snap action only.
When a new board 1xe2x80x2 and a previously installed board 1 are to be joined together along their long sides 4a, 4b as shown in FIGS. 1a-1c, the long side 4b of the new board 1xe2x80x2 is pressed against the long side 4a of the previous board 1 as shown in FIG. 1a, so that the locking tongue 20 is introduced into the recess 16. The board 1xe2x80x2 is then angled downwards towards the subfloor 12 according to FIG. 1b. In this connection, the locking tongue 20 enters the recess 16 completely, while the locking element 8 of the strip 6 enters the locking groove 14. During this downward angling the upper part 9 of the locking element 8 can be operative and provide guiding of the new board 1xe2x80x2 towards the previously installed board 1. In the joined position as shown in FIG. 1c, the boards 1, 1xe2x80x2 are locked in both the direction D1 and the direction D2 along their long sides 4a, 4b, but the boards 1, 1xe2x80x2 can be mutually displaced in the longitudinal direction of the joint along the long sides 4a, 4b. 
FIGS. 2a-2c show how the short sides 5a and 5b of the boards 1, 1xe2x80x2 can be mechanically joined in the direction D1 as well as the direction D2 by moving the new board 1xe2x80x2 towards the previously installed board 1 essentially horizontally. Specifically, this can be carried out subsequent to joining the long side of the new board 1xe2x80x2 to a previously installed board 1 in an adjoining row by means of the method according to FIGS. 1a-1c. In the first step in FIG. 2a, bevelled surfaces adjacent to the recess 16 and the locking tongue 20 respectively cooperate such that the strip 61 is forced to move downwards as a direct result of the bringing together of the short sides 5a, 5b. During the final bringing together of the short sides, the strip 6xe2x80x2 snaps up when the locking element 8xe2x80x2 enters the locking groove 14xe2x80x2, so that the operative locking surfaces 10, 10xe2x80x2 of the locking element 8xe2x80x2 and of the locking groove 14xe2x80x2 will engage each other.
By repeating the steps shown in FIGS. 1a-c and 2a-c, the whole floor can be laid without the use of glue and along all joint edges. Known floorboards of the above-mentioned type are thus mechanically joined usually by first angling them downwards on the long side, and when the long side has been secured, snapping the short sides together by means of horizontal displacement of the new board 1xe2x80x2 along the long side of the previously installed board 1. The boards 1, 1xe2x80x2 can be taken up in the reverse order of laying without causing any damage to the joint, and be laid again. These laying principles are also applicable to the present invention.
For optimal function, subsequent to being joined together, the boards should be capable of assuming a position along their long sides in which a small play can exist between the operative locking surface 10 of the locking element and the operative locking surface 10xe2x80x2 of the locking groove 14. Reference is made to WO 94/26999 for a more detailed description of this play.
In addition to what is known from the above-mentioned patent specifications, a licensee of Vxc3xa4linge Aluminium AB, Norske Skog Flooring AS, Norway (NSF), introduced a laminated floor with mechanical joining according to WO 94/26999 in January 1996 in connection with the Domotex trade fair in Hannover, Germany. This laminated floor, which is marketed under the trademark Alloc(copyright), is 7.2 mm thick and has a 0.6-mm aluminium strip 6 which is mechanically attached on the tongue side. The operative locking surface 10 of the locking element 8 has an inclination (hereinafter termed locking angle) of about 80xc2x0 to the plane of the board. The vertical connection is designed as a modified tongue-and-groove joint, the term xe2x80x9cmodifiedxe2x80x9d referring to the possibility of bringing the tongue groove and tongue together by way of angling.
WO 97/47834 (owner Unilin Beeher B.V., the Netherlands) describes a strip-lock system which has a fibreboard strip and is essentially based on the above known principles. In the corresponding product, xe2x80x9cUniclic(copyright)xe2x80x9d, which this owner began marketing in the latter part of 1997, one seeks to achieve biasing of the boards. This results in high friction and makes it difficult to angle the boards together and to displace them. The document shows several embodiments of the locking system. The xe2x80x9cUniclic(copyright)xe2x80x9d product is shown in section in FIG. 4b. 
Other known locking systems for mechanical joining of board materials are described in, for example, GB-A2,256,023 showing unilateral mechanical joining for providing an expansion joint in a wood panel for outdoor use, and in U.S. Pat. No. 4,426,820 (shown in FIG. 4d) which concerns a mechanical locking system for plastic sports floors, which floor is intentionally designed in such manner that neither displacement of the floorboards along each other nor locking of the short sides of the floorboards by snap action is allowed.
In the autumn of 1998, NSF introduced a 7.2-mm laminated floor with a strip-lock system which comprises a fibreboard strip and is manufactured according to WO 94/26999 and WO 99/66151. This laminated floor is marketed under the trademark xe2x80x9cFiboloc(copyright)xe2x80x9d and has the cross-section illustrated in FIG. 4a. 
In January 1999, Kronotex GmbH, Germany, introduced a 7.8 mm thick laminated floor with a strip lock under the trademark xe2x80x9cIsilock(copyright)xe2x80x9d. A cross-section of the joint edge portion of this system is shown in FIG. 4c. Also in this floor, the strip is composed of fibreboard and a balancing layer.
During 1999, the mechanical joint system has obtained a strong position on the world market, and some twenty manufacturers have shown, in January 2000, different types of systems which essentially are variants of Fiboloc(copyright), Uniclic(copyright) and Isilock(copyright).
Although the floor according to WO 94/26999 and WO 99/66151 and the floor sold under the trademark Fiboloc(copyright) exhibit major advantages in comparison with traditional, glued floors, further improvements are desirable mainly in thin floor structures.
The joint system consists of three parts. An upper part P1 which takes up the load on the floor surface in the joint. An intermediate part P2 that is necessary for forming the vertical joint in the D1 direction in the form of tongue and tongue groove. A lower part P3 which is necessary for forming the horizontal lock in the D2 direction with strip and locking element.
In thin floorboards, it is difficult to provide, with prior-art technique, a joint system which at the same time has a sufficiently high and stable upper part, a thick, strong and rigid tongue and a sufficiently thick strip with a high locking element. Nor does a joint system according to FIG. 4d, i.e. according to U.S. Pat. No. 4,426,820, solve the problem since a tongue groove with upper and lower contact surfaces which are parallel with the upper side of the floorboard or the floor plane, cannot be manufactured using the milling tools which are normally used when making floorboards. The rest of the joint geometry in the design according to FIG. 4d cannot be manufactured by working a wood-based board since all surfaces abut each other closely, which does not provide space for manufacturing tolerances. Moreover, strip and locking elements are dimensioned in a manner that requires considerable modifications of the joint edge portion that is to be formed with a locking groove.
At present there are no known products or methods which afford satisfactory solutions to problems that are related to thin floorboards with mechanical joint systems. It has been necessary to choose compromises which (i) either result in a thin tongue and sufficient material thickness in the joint edge portion above the corresponding tongue groove in spite of plane-parallel contact surfaces or (ii) use upper and lower contact surfaces angled to each other and downwardly extending projections and corresponding recesses in the tongue and the tongue groove respectively of adjoining floorboards or (iii) result in a thin and mechanically weak locking strip with a locking element of a small height.
Therefore an object of the present invention is to obviate this and other drawbacks of prior art. Another object of the invention is to provide a locking system, a floorboard, and a method for making a floorboard having such a locking system, in which it is at the same time possible to obtain
(i) a stable joint with tongue and tongue groove,
(ii) a stable portion of material above the tongue groove,
(iii) a strip and a locking element, which have high strength and good function.
To achieve these criteria simultaneously, it is necessary to take the conditions into consideration which are present in the manufacture of floorboards with mechanical locking systems. The problems arise mainly when laminate-type thin floorboards are involved, but the problems exist in all types of thin floorboards. The three contradictory criteria will be discussed separately in the following.
(i) Tongue-and-Groove Joint
If the floor is thin there is not sufficient material for making a tongue groove and a tongue of sufficient thickness for the intended properties to be obtained. The thin tongue will be sensitive to laying damage, and the strength of the floor in the vertical direction will be insufficient. If one tries to improve the properties by making the contact surfaces between tongue and tongue groove oblique instead of parallel with the upper side of the floorboard, the working tools must during working be kept extremely accurately positioned both vertically and horizontally relative to the floorboard that is being made. This means that the manufacture will be significantly more difficult, and that it will be difficult to obtain optimal and accurate fitting between tongue and tongue groove. The tolerances in manufacture must be such that a fitting of a few hundredths of a millimetre is obtained since otherwise it will be difficult or impossible to displace the floorboards parallel with the joint edge in connection with the laying of the floorboards.
(ii) Material Portion Above the Tongue Groove
In a mechanical locking system glue is not used to keep tongue and tongue groove together in the laid floor. At a low relative humidity the surface layer of the floorboards shrinks, and the material portion that is located above the tongue groove and consequently has no balancing layer on its underside, can in consequence be bent upwards if this material portion is thin. Upwards bending of this material portion may result in a vertical displacement between the surface layers of adjoining floorboards in the area of the joint and causes an increased risk of wear and damage to the joint edge. To reduce the risk of upwards bending, it is therefore necessary to strive to obtain as thick a material portion as possible above the tongue groove. With known geometric designs of locking systems for mechanical joining of floorboards, it is then necessary to reduce the thickness of the tongue and tongue groove in the vertical direction of the floorboard if at the same time efficient manufacture with high and exact tolerances is to be carried out. A reduced thickness of tongue and tongue groove, however, results in, inter alia, the drawbacks that the strength of the joint perpendicular to the plane of the laid floor is reduced and that the risk of damage caused during laying increases.
(iii) Strip and Locking Element
The strip and the locking element are formed in the lower portion of the floorboard. If the total thickness of a thin floorboard is to be retained and at the same time a thick material portion above the locking groove is desirable, and locking element and strip are to be formed merely in that part of the floorboard which is positioned below the tongue groove, the possibilities of providing a strip having a locking element with a sufficiently high locking surface and upper guiding part will be restricted in an undesirable manner. The strip closest to the joint plane and the lower part of the tongue groove can be too thick and rigid and this makes the locking by snap action by backwards bending of the strip difficult. If at the same time the material thickness of the strip is reduced and a large part of the lower contact surface is retained in the tongue groove, this results on the other hand in a risk that the floorboard will be damaged while being laid or subsequently removed.
A problem that is also to be taken into consideration in the manufacture of floorboards, in which the components of the locking systemxe2x80x94tongue/tongue groove and strip with a locking element engaging a locking groovexe2x80x94are to be made by working the edge portions of a board-shaped starting material, is that it must be possible to guide the tools in an easy way and position them correctly and with an extremely high degree of accuracy in relation to the board-shaped starting material. Guiding of a chip-removing tool in more than one direction means restrictions in the manufacture and also causes a great risk of reduced manufacturing tolerances and, thus, a poorer function of the finished floorboards.
To sum up, there is a great need for providing a locking system which takes the above-mentioned requirements, problems and desiderata into consideration to a greater extent than prior art. The invention aims at satisfying this need.
These and other objects of the invention are achieved by a locking system, a floorboard, a floor and a manufacturing method having the features stated in the independent claims. The dependent claims define particularly preferred embodiments of the invention.
The invention is based on a first understanding that the identified problems must essentially be solved with a locking system where the lower contact surface of the tongue groove is displaced downwards and past the upper part of the locking element.
The invention is also based on a second understanding which is related to the manufacturing technique, viz. that the tongue groove must be designed in such manner that it can be manufactured rationally and with extremely high precision using large milling tools which are normally used in floor manufacture and which, during their displacement relative to the joint edge portions of the floorboard that is to be made, need be guided in one direction only to provide the parallel contact surfaces while the tool is displaced along the joint edge portion of the floorboard material (or alternatively the joint edge portion is displaced relative to the tool). In known designs of the joint edge portions, such working requires in most cases guiding in two directions while at the same time a relative displacement of tool and floorboard material takes place.
According to a first aspect of the invention, a locking system is provided of the type which is stated by way of introduction and which according to the invention is characterised by the combination by the combination
that the upper and lower contact surfaces are essentially plane-parallel and extend essentially parallel with a plane containing the upper side of the floorboards, and
that the upper edge of the locking element, which upper edge is closest to a plane containing the upper side of the floorboards, is located in a horizontal plane, which is positioned between the upper and the lower contact surfaces but closer to the lower than the upper contact surfaces.
According to another aspect of the invention, a new manufacturing method for making strip and tongue groove is provided. According to conventional methods, the tongue groove is always made by means of a single tool. The tongue groove according to the invention is made by means of two tools in two steps where the lower part of the tongue groove and its lower contact surface are made by means of one tool and the upper part of the tongue groove and its upper contact surface are made by means of another tool. The method according to the invention comprises the steps 1) of forming part of the strip, part of the lower part of the tongue groove and the lower contact surface by means of an angled milling tool operating at an angle  less than 90xc2x0 to the horizontal plane of the floorboard and the strip, and 2) forming the upper part of the tongue groove and the upper contact surface by means of a separate horizontally operating tool.
According to another aspect of the invention, also a method for making a locking system and floorboards of the above type with plane-parallel upper and lower contact surfaces is provided. This method is characterised in
that parts of said tongue groove and at least parts of the lower contact surface are formed by means of a chip-removing tool, whose chip-removing surface portions are brought into removing contact with the first joint portion and are directed obliquely inwards and past said joint plane and
that the upper contact surface and parts of the tongue groove are formed by means of a chip-removing tool, whose chip-removing surface portions are moved into removing contact with the first joint portion in a plane which is essentially parallel with a plane containing the upper side of the floorboard.